1. Field of the Invention
This invention relates to apparatus and a method for controlling a defrost cycle for effecting defrost of an ice accumulating heat exchanger. More specifically, the present invention concerns initiating a defrost cycle based on the phase angle of the electric motor powering the compressor in a refrigeration circuit.
2. Prior Art
Air conditioners, refrigerators and heat pumps produce a controlled heat transfer by the evaporation in a heat exchanger of a liquid refrigerant under appropriate pressure conditions to produce desired evaporator temperatures. Liquid refrigerant removes its latent heat of vaporization from the medium being cooled and in this process is converted into a vapor at the same pressure and temperature. This vapor is then conveyed into a compressor wherein its temperature and pressure are increased. The vapor then is conducted to a separate heat exchanger serving as a condenser wherein the gaseous refrigerant absorbs its heat of condensation from a heat transfer fluid in heat exchange relation therewith and changes state from a gas to a liquid. The liquid is supplied to the evaporator after flowing through an expansion device which acts to reduce the pressure of the liquid refrigerant such that the liquid refrigerant may evaporate within the evaporator to absorb its heat of vaporization and complete the cycle.
The specific application incorporating this refrigeration circuit utilizes a heat exchanger serving as an evaporator wherein the evaporator is either located in ambient air at a temperature below the freezing point of water or wherein the heat exchanger itself has a surface temperature below the freezing point of water. In either event, as air is circulated over the heat exchanger, water vapor in the air is condensed and frozen on the surfaces of the heat exchanger. As the frost accumulates on the heat exchanger a layer of ice is built up between the portion of the heat exchanger carrying refrigerant and the air flowing thereover. This layer of ice acts as an insulating layer inhibiting the heat transfer between refrigerant and air. Additionally, the ice may serve to block narrow air flow passageways between fins utilized to enhance heat transfer. This additional effect further serves to reduce heat transfer since lesser amounts of air will be circulated in heat exchange relation with the refrigerant carrying conduits.
To effectively utilize a refrigeration circuit wherein the evaporator may encounter these conditions, such as a heat pump operating in relatively low outdoor ambient air conditions or the evaporator of a refrigeration circuit for a cold room, it is necessary to provide apparatus for removing the accumulated frost. Many conventional methods are known such as supplying electric resistance heat, reversing the heat pump such that the evaporator becomes a condenser or other refrigerant circuiting techniques to direct hot gaseous refrigerant directly to the frosted heat exchanger.
Many of these defrost techniques utilize energy that is not effectively used for transferring heat energy to a space to be conditioned or to another end use served by the entire system. To reduce the amount of heat energy wasted or otherwise consumed in the defrost operation it is a design selection to utilize a defrost system which places the refrigeration circuit in the defrost mode only when needed.
Different types of control systems have been utilized for initiating defrost. A combination of a timer and a thermostat may be used to determine when to initiate defrost. The timer periodically checks to see whether or not the evaporator temperature or a temperature dependent thereon is below a selected level, and if so acts to place the system in defrost. Other types of prior art defrost initiation systems have included measuring infrared radiation emitted from the fins of the refrigerant carrying coil, measuring the air pressure differentials of the air flow flowing through the heat exchanger, measuring multiple independent variables that would indicate icing, utilizing an electrical device placed on the fin whose characteristics change depending on the temperature of the device, optical-electrical methods and other methods involving the monitoring of various electrical parameters.
The present invention is directed towards sensing the phase angle of the motor driving the compressor. It has been found that as the load on the electric motor decreases the power factor of the motor decreases and the phase angle increases. The phase angle, as used herein, shall refer to the difference in angle between the voltage supplied to the compressor and the current flowing through the compressor. Hence, as the load on the compressor varies, the phase angle will vary. By determining the appropriate phase angle at a frost free condition the maximum load on the refrigeration circuit is determined. As the heat exchanger accumulates frost the load on the compressor motor decreases since the amount of heat energy that is being transferred decreases due to the inefficiencies developing in the heat exchanger having the frost accumulated thereon. As the load on the compressor decreases the load on the compressor motor decreases and consequently its phase angle increases. By comparing the change in the phase angle between the frost free condition as compared to the sensed condition an appropriate differential therebetween may be utilized to initiate defrost.